Ni Micro Research Articles

Improved shear property of Sn-3.0Ag-0.5Cu/Ni micro solder joints under thermal shock between 77 K and 423 K by adding TiO2 nanoparticles

The rapid development of space exploration technology was intensifying the desire for lead-free solders with high performance and reliability under extreme temperature environments. In this study, the influence of TiO2 nanoparticle addition on the microstructure, growth behavior of interfacial intermetallic compounds (IMCs) and shear property of Sn-3.0Ag-0.5Cu (SAC305)/Ni micro solder joints under thermal shock from 77 K to 423 K was systematically investigated. The results indicated that the grain size of β-Sn, IMC (Ag3Sn and (Cu,Ni)6Sn5) particles inside the solder was refined by adding 0.05 and 0.1 wt% TiO2 nanoparticles. Due to the spalling of interfacial IMCs into the solder occurring in all micro joints during thermal shock, the thickness of interfacial IMCs did not show a monotony increasing trend with increasing thermal cycles. However, the micro composite solder joint added with 0.05 and 0.1 wt% TiO2 nanoparticles always exhibited thinner interfacial IMC thickness and higher shear force compared with the plain micro joints during thermal shock. The inhibiting effect of TiO2 nanoparticles on the interfacial IMC growth could be explained by the adsorption theory and grain boundary pinning theory. The improvement in shear property of micro composite solder joints was primarily induced by the refinement of IMC particles in the solder and the reduced growth rate of interfacial IMCs by adding TiO2 nanoparticles.

Optimization of forward pulsed currents for combining the precision shaping and polishing of nickel micro mould tools to reduce demoulding defects

Precise tooling is vital for defect-free production of micro injection moulded (μ-IM) or hot-embossed products. The demoulding stage of such moulding and forming processes poses a serious challenge to the integrity of thin miniature features because of friction, adhesion, and thermal stresses. Typically, micro moulds involve geometrically textured patterns or features such as linear ridges, pillars, channels, and holes, the characteristic dimensions of which range from 10 to 300 μm. Realistically complex mould designs, containing precision micro features (enhanced fillet radius and positive draft angle) and high surface quality, are presented in this work. Electropolishing based on forward pulse currents (PC) has been used to shape and polish Ni micro moulds that contain sets of micron-scaled linear ridges and star patterns in order to ease the separation of moulded polymeric parts from the metallic mould during ejection and demoulding. The use of forward pulsed currents improved the mould design by increasing the fillet radii and draft angle while keeping the surface roughness low and maintaining a good surface shine. An optimization study of forward PC using a green solution of nickel sulfamate varied EP times (0–70 min) and duty cycles (40, 50, 60, and 70%) at a process conditions of 2.8 V, 50 °C, and 250 rpm. The best topographical and morphological changes were observed for a typical microfluidic channel (w × h, 100 × 110 μm) with an EP time of 70 min and 50% duty cycle: fillet radius increased by 3.8 μm, draft angle by 3.3°, and the channel width reduced by 11.4% while surface roughness changed by 8.6% and surface shine improved by 48.9%. Experimental validation was performed using hot embossing wherein the electropolished Ni mould replicated the micro channels and star patterns in PMMA chips with notably fewer burrs, material pile up, and no feature distortion. Moreover, there was a reduction in the side wall roughness of micro channels in PDMS casting with electropolished Ni mould by 16%. Hence, this work presents a significant scientific contribution to improving the efficiency of micro mould tools and reduces the defects caused by friction and adhesion in replicated polymeric parts.

Enhancement of micro hole wetting and hydrogen bubble release in the process of Ni micro electroforming under megasonic field

Micro hole wetting and hydrogen bubble release are two technical issues in the process of micro hole electroforming. A novel method of megasonic (1.7 MHz) field was introduced in order to solve the two problems and fabricate 20 μm micro column array in this paper. Micro electroforming experiments, capillary tube experiments, electrochemical experiments and testing of contact angle and surface tension coefficient were conducted without and with megasonic field. Experimental results indicated that megasonic field can promote micro hole wetting and bubbles release in the process of micro electroforming. 20 μm micro column array can be fabricated successfully under 2 W/cm2 megasonic field. Because megasonic can decrease the contact angle between photoresist and electroforming solution, reduce the surface tension coefficient of electroforming solution and increase equilibrium potential of nickel. New mechanisms on enhancement of micro hole wetting and bubble release under megasonic field were revealed. The research provides a new comprehension of micro hole wetting and bubble release, which may be good for develop a new method to fabricate micro column array.

Raman spectroscopy mapping of Si (001) surface strain induced by Ni patterned micro arrays

Patterned arrays of nickel microstructures were fabricated by electron beam lithography and DC sputtering on the surface of Si (001) substrates. Influence of the micro-arrays on the Si substrate surface was evaluated using polarized micro-Raman spectroscopy. Raman spectra maps were taken in two configurations z¯(xx)z and z¯(yy)z for arrays of 18.5 μm × 15.5 μm. The Si longitudinal optical (LO) phonon shifted to lower vibrational frequencies by 5 cm−1 in regions near to the vicinities of the Ni micro structures. From the deformation elastic theory, it is found that this frequency shift is explained by an in-plane tensile stress induced by the Ni micro arrays producing a strain of 1.4%.

Residual stress modeling and analysis for micro electroforming layer

Large residual stress in the micro electroforming layer has been considered to be the main reason for causing warpage, delamination and micro cracking of the microdevice. These undesirable phenomena dramatically influence the productivity, dimensional accuracy and service life of the microdevice. Reasonable control of the residual stress is a direct and efficient way to improve the production rate of microdevice and expand the application of microdevice. In this paper, the residual stress finite element analysis (FEA) simulation model for Ni micro electroforming layer was established by the method of “Equivalent Reference Temperature (ERT)”. The simulation result shows that the ERT for Ni micro electroforming layer is 238.15 K. The error of residual stress value is 0.37% between the simulation result and the experimental result. This error value proves the accuracy of the FEA model.

Flexible Field Emitter for X-ray Generation by Implanting CNTs into Nickel Foil.

This paper reports on a flexible Ni micro wire with CNTs embedded into its surface. By using micromachining technology, for the first time, we could implant nanoscale materials into micro-scale metal substrate at room temperature. Thanks to the effective direct contact and the strong interactions between CNTs and the substrate, field emission current of 1.11 mA (current density of 22.2 mA/cm2) could be achieved from the micro wire. Moreover, the wire shows excellent mechanical properties for large amplitude bending, which is beneficial for geometric designing. To check the practical application of the wire, a simplified X-ray imaging system was set up by modifying a conventional tube. The gray shade that appears on the sensitive film after being exposed to the radiation confirms the X-ray generation.

Double-stage sintering behavior of a nickel nanoparticle dispersed micro powder

Powder metallurgy (PM) has undergone innovations through powder advances and novel process development. Another option for advancement of PM technology is exploitation of the extreme bimodality of nano/micro powder mixtures. The present study describes bimodal powder preparation involving dispersion of nickel nanoparticles onto the surface of a micro powder to facilitate the compaction ability and sinterability of the Ni micro powder. The bimodal powder is designed by synthesizing Ni nanoparticles by reactive RF thermal plasma from a Ni(OH)2 micro powder, selecting a spiky-shaped Ni micro powder as the supporting powder, and then filling the pockets on the surface of the micro powder with the nanoparticles. Simple 3-D turbulent mixing successfully produces spheroidized and smoothened micro-granules consisting of well-dispersed nanoparticles on the surface of the micro powder. Nanoparticle compaction is usually difficult, but the compaction behavior of the micro-granules is similar to that of the micro powder. The effect of nanoparticle addition on sinterability, investigated by dilatometry, includes double-stage sintering behavior of the micro-granule powder, whereas the spiky-shaped micro powder shows a typical monotonous sintering behavior. Consequently, the first shrinkage at low temperature arises from the enhanced sinterability of the nanoparticles, while the second shrinkage shows similar behavior to the micro powder. Within the scope of this study, the micro-granules show a higher relative density than the micro powder.

NiO nanosteps on Ni: wide band gap p-type nanostructure for efficient cold cathode and magnetically separable photocatalyst

Pure nickel micro particles were chemically treated with various concentrations of NaOH and were subsequently annealed in 800 °C. The dimensions of the resulting nanosteps were tuned by simply varying the annealing time durations. Formation of Ni-NiO composite phase was confirmed by x-ray diffraction studies. The morphology of the as-prepared samples was investigated by field emission scanning electron microscope whereas the transmission electron microscope revealed the Ni core NiO shell structure of the samples. The NiO nanosteps showed much improved field emission properties compared to pure Ni micro particles and pure NiO powder synthesized by simply annealing the Ni particles. Finite element based simulation studies revealed strong enhancement of the electric field in the NiO nanostep samples and the simulated results were compared with the experimental outcome. The photocatalytic activities of the as-prepared samples were also investigated, which showed that Ni-NiO nanosteps is a magnetically separable photocatalyst.

Enhanced Ni3Sn4 nucleation and suppression of metastable NiSn3 in the solid state interfacial reactions between Sn and cone-structured Ni

Interfacial reactions of Ni micro cones with Sn at solid state temperatures from 373 K to 473 K were studied and compared with Sn/flat Ni diffusion couples. Due to its morphological characteristics, cone-structured Ni enhanced the nucleation of Ni3Sn4, resulting in thicker Ni3Sn4, which is also smaller in grain size and different in texture. At lower annealing temperatures (<423 K), cone-Ni also strongly suppresses the nucleation of metastable NiSn3. The stripe-like micromorphology and special texture of Ni cones are believed to be the reasons for enhanced Ni3Sn4 nucleation.

Fabrication of Metal Micro Needle Array by LIGA Process

Polymer materials, such as PMMA and PDMS, have been of great interest in the research and development of MEMS recently due to their relatively low cost and easy process. In this research, we are trying to fabricate Ni Micro needle array from PMMA master by through PDMS mold based on multi-mold transformation technology. The fabricated micro nickel needles array is 900 microneedles in 1 cm2 chip area and the height of each needle is about 150um.

Fabrication of ultrafine hollow Ni and Ni/Fe fibers and their dispersion characteristics in the epoxy matrix

Abstract In order to develop a material with light weight and homogeneous characteristics for the electromagnetic wave absorbing, hollow Ni and Fe metallic micro fibers were fabricated by electroless plating and heat treatment method. It is expected that hollow and fibrous structure of fibers can reduce the weight and increase the efficiency of electromagnetic properties. As a substrate for electroless metal plating, commercial polyester micro fibers were used. The surface of micro fibers was coated with Ni and Ni/Fe metal layers and then heat-treated under the argon atmosphere to make the fiber a hollow structure. The surface morphology and composition were verified by SEM and EDS analyses. Hollow structures of the fibers were observed and the optimal thickness of Ni and Fe layers were about 200–300 and 500–700 nm, respectively. Polymeric composites containing 10, 20 and 30 wt.% of hollow Ni/Fe micro fibers were fabricated by mechanical mixing method. SEM images demonstrated the uniform distribution of hollow metallic micro fibers in the epoxy matrix.

Ultrasonic Synthesis of Nickel Nanostructures on Glassy Carbon Microspheres and Their Application for Ethanol Electrooxidation

Nickel nanostructures were synthesized onto the surface of glassy carbon microspheres (10-20 microm) with the aid of ultrasounds to produce bulk quantities of nickel micro and nanostructure surface-modified carbon microspheres (Ni-CMs). This new composite material was characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and cyclic voltammetry (CV). The synthesized Ni micro and nanostructures on the glassy carbon microspheres were found to possess a large proportion of Ni (111) planes. The material was proved to be of interest for electrochemical ethanol sensing in particular. An optimized limit of detection of 1.3 ppm was obtained for ethanol in aqueous solution.

Templated Electrodeposition of Aspect-Ratio Adjusted Ni Micro- and Nanorods and their Magnetically Driven Diffusion through an Extracellular Matrix

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Inelastic Constitutive Relationship of Electroplated Nickel Films at Elevated Temperatures for Design of Micro Connector

This paper describes elevated temperatures tensile strength and inelastic constitutive relationship of electroplated nickel (Ni) thin films used in high-density micro connectors. The compact tensile tester operated in Scanning Probe Microscope (SPM) evaluated mechanical properties of electroplated Ni specimens, with nominal dimensions of 15 μm in thickness, 50 μm in width and 600 μm in length, at elevated temperatures. All specimens have line patterns on their specimen gauge section to measure axial elongation under tensile loading with an SPM cantilever. Strain rates faster than 5 %/sec were employed to obtain the time-independent Young's modulus and yield stress of the Ni specimens. Averaged Young's modulus was closed to that of a bulk of Ni, however ultimate tensile strength showed a larger value. Yield stress and breaking elongation were also quite different from previous reported values of the bulk. Inelastic constitutive equations were proposed as a function of temperature in order to reveal inelastic deformation behavior of the Ni micro connector at elevated temperatures in finite element analyses.

Development of Ni Grid Mask for Microfocus X-ray Inspection Apparatus

A Microfocus X-ray inspection apparatus requires a grid mask as a scale which gives an indication of distortion correction and magnification. In addition, a grid mask must be an X-ray absorber. A thick micro structure is available to an X-ray absorber. Ni which is a typical electroforming material has a vocation for a thick micro structure. In this study, we make use of Ni micro structure 40μm thick as X-ray absorber. We fabricated the Ni grid mask using LIGA-like process. When the X-ray tube voltage was 70kV, we could observe the plain gird through an IC chip. Additionally, the center distance of the grid was to within an average of ±1μm in both the lengthwise direction and crosswise direction. These results suggest a Ni micro-structure is available as the position indicator for a Microfocus X - ray Inspection Systems.

Magnetic Domain Imaging of Ni Micro Ring and Micro Dot array by Photoelectron Emission Microscopy

The magnetic domain structures of Ni micro ring (the inner and outer diameters are changed in a micrometer region) and micro dot array with different shapes and with a small separation between dots were observed by the combined method of photoelectron emission microscopy and X-ray magnetic circular dichroism. For the micro rings with a large inner diameter, a flux-closure (FC) domain structure tends to appear more easily than for those with a small inner diameter. This means that the FC domain structure of the micro rings with a large inner diameter is energetically more stable than those of the micro rings with a small inner diameter and the disks. After applying an external pulsed magnetic field of about 40 mT in the in-plane direction, each domain structure switched to an almost perfect single-domain structure. Because of the absence of a central vortex core in the ring as compared with the disks, exchange energy decreases substantially. These phenomena are discussed along with the simulation results. For the micro dot array, the interaction between dots was directly observed. Particularly for the circular disks with a small diameter showing FC domains, no interaction is observed due to the lack of stray fields from each disk. On the other hand, dots with multi domain structures interacted. These types of information may provide us fundamental knowledge for designing high-density magnetic recording media.

Dynamic actuation properties of TiNi shape memory diaphragm

In order to realize a micropump with a shape memory alloy (SMA) diaphragm actuator, TiNi thin film of about 7 μm in thickness was deposited by flash evaporation and its dynamic deformation–shape recovery properties were studied using a bulge test. The TiNi diaphragm was deformed by applying a gas pressure of 200 kPa, heated resistively to recover its initial flat shape, and then air-cooled to achieve deformation once again. During this thermal cycle, temperature and deflection of the diaphragm were monitored at its center. In order to monitor temperature, we fabricated a Cu–Ni micro thermocouple on the diaphragm by conventional evaporation. When the diaphragm was heated, shape recovery occurred at about 60°C. This continued after the temperature for termination of reverse martensitic transformation, Af (about 70°C), had been reached. Moreover, when the diaphragm was air-cooled, redeformation began even at temperatures higher than that for the commencement of martensitic transformation, Ms (about 60°C). From a FEM simulation and temperature measurements taken using thermography, these results could be explained by the temperature gradient formed in the diaphragm due to thermal conduction. When heating rate was increased, time required to complete shape recovery decreased and maximum displacement for shape recovery increased. This could also be explained in terms of the temperature gradient.